Histologic conversion of nodular lymphomas to more aggressive patterns has been well described and occurs in 15% to 40% of cases. Most conversions have involved changes from nodular to diffuse patterns and often from small to larger cell types. Conversions to undifferentiated lymphomas, but not specifically to the Burkitt's type, have infrequently been described. This report describes three cases of nodular poorly differentiated lymphocytic lymphoma that converted to a Burkitt's-like lymphoma. Conversion was associated with short survivals, increasing lactate dehydrogenase (LDH) levels, and changes in DNA stemline, RNA content, and S-phase values, as determined by flow cytometry. Karyotypes in two cases revealed a t(14;18). Surface immunoglobulin could be demonstrated on transformed Burkitt's-like cells from lymph node in one case, but in none of the cases on cells from bone marrow.